This invention relates generally to the preparation of low hysteresis rubber by termination with a sulfur-containing reagent.
It is often desirable to produce elastomeric polymers capable of exhibiting reduced hysteresis when properly compounded with other ingredients such as reinforcing agents and then vulcanized. Such elastomers, when fabricated into components for constructing articles such as tires, vibration isolators, power belts, and the like, will manifest properties of increased rebound, decreased rolling resistance and reduced heat-build up when subjected to mechanical stress during normal use.
The hysteresis of an elastomers refers to the difference between the energy applied to deform an article made from the elastomer and the energy released as the elastomer returns to its initial, un-deformed state. In pneumatic tires for instance, lowered hysteretic properties are associated with reduced rolling resistance and reduced heat build-up during operation of the tire. These properties, in turn, result in lowered fuel consumption of vehicles using such tires and prolonged tire life. In such contexts, the property of lowered hysteresis of compounded, vulcanizable elastomer compositions is particularly significant.
Examples of such compounded elastomer systems are known to the art and are comprised of at least one elastomer (that is, a natural or synthetic polymer exhibiting elastomeric properties, such as a rubber), a reinforcing filler agent (such as finely divided carbon black, thermal black, or mineral fillers such as clay, silica and the like) and curing with a sulfur-containing vulcanizing system.
Various synthetic strategies have been developed to provide elastomers with molecular structures exhibiting reduced hysteresis. One technique is to produce elastomers of very high molecular weight. In such high molecular weight systems, the number of free, unattached molecular chain-ends per given weight in the vulcanizates made from them are reduced. Because the presence of free, unbound chain ends is believed to be a significant factor in hysteretic energy loss (because they cannot participate in the elastic recovery processes), reducing their number is believed to lead to a desirable reduction in hysteretic energy loss. A reduction in the measured tan xcex4 of the elastomer is indicative of a reduction in the hysteresis of the elastomer.
Another approach to producing elastomers with reduced hysteresis properties involves xe2x80x9cjumpingxe2x80x9d of elastomer intermediates having a terminal functionality which is reactive under anionic polymerization conditions. Such jumping reactions join two elastomer molecules at their functional ends to produce a single molecule of much higher molecular weight. Such participation again reduces the number of free, unbound chain ends in the vulcanizate which results in low hysteretic energy loss.
It has also been recognized that carbon black, employed as a reinforcing filler in rubber compounds, should be well dispersed and separated throughout the rubber in order to improve various physical properties. One physical property that is improved by this dispersion and separation is a lowered level of hysteresis in the resultant vulcanizate. This improved dispersion may be achieved, for example, by reacting a metal terminated polydiene with a capping agent, such as a halogenated nitrile, a heterocyclic aromatic nitrogen-containing compound or a trialkyl tin halide. Additionally, it is known in the art that both ends of the polydiene chains can be capped with polar groups by utilizing functionalized anionic initiators, such as lithium amide or lithium trialkyl tin halides.
In another approach to reducing hysteresis, lithium amino magnesiate anionic polymerization initiators, stable at high polymerization temperatures, have been employed to produce polymers containing a high level of tertiary amine functionality with functional end groups derived from the initiator. Such polymers can be compounded to produce vulcanizable elastomers exhibiting reduced hysteresis properties.
It has also been known to produce modified elastomers with purely hydrocarbyl terminal functionality which are capable of conferring low hysteresis properties. For example, commonly assigned U.S. patent application Ser. No. 07/636,961 describes elastomers with tin containing end-groups derived by initiating polymerization under anionic conditions with tin-lithium compounds such as trialkyl tin (IV) lithium, that is, (alkyl)3SnLi groups.
Another technique is to prepare elastomer molecules with end groups capable of interacting with the reinforcing fillers, such as carbon black, present in compounded elastomer compositions. Also, such interaction with carbon black is thought to coat the aggregated carbon black and reduce formation of the carbon black network. Such interactive end groups include those derived from various metal reagents as well as those derived from polar organic reagents such as amines, amides, esters, imines, imides, ketones and various combinations of such groups. One example of functional end-capping is provided in published European Patent Application Number EP 0 316 255A2 which discloses a process for end-capping polydienes by reacting a metal terminated polydiene with a capping agent such as a halogenated nitrile, a heterocyclic aromatic nitrogen containing compound or an alkyl benzoate. Additionally, the application discloses that both ends of the polydiene chains can be capped with polar groups by utilizing functionalized initiators, such as lithium amides.
End functionalized polymers, also known as telechelic and semi-telechelic polymers, are industrially important polymers and pre-polymers in their own right. They are used for preparing segmented block copolymers and crosslinked materials and, are also useful in preparing graft copolymers.
Still other strategies aimed at preparing reduced hysteresis compounds have included high temperature mixing of the filler-rubber mixtures in the presence of selectively reactive promoters to promote compounding material reinforcement, surface oxidation of the compounding materials, and chemical modifications to the terminal end of polymers using 4,4xe2x80x2-bis(diethylamino)-benzophenone (Michler""s ketone), tin coupling agents and the like, and surface grafting thereon. All of these approaches have focused upon increased interaction between the elastomer and the compounding materials.
Use of organolithium initiators to polymerize conjugated diene, triene, and monovinyl aliphatic and aromatic monomers is known in the art. These polymerizations proceed according to anionic polymerization mechanisms. That is, these polymerization reactions generally include the reaction of monomers by nucleophilic initiation to form and propagate a polymeric structure. Throughout the formation and propagation of this polymer, the polymeric structure is ionic or xe2x80x9clivingxe2x80x9d. A living polymer, therefore, is a polymeric segment having at least one living or reactive end. For example, when a lithium containing initiator is employed to initiate the formation of a polymer, the reaction will produce a reactive polymer having a lithium atom at its living or reactive end.
Organolithium initiators are known in the art. Initiators which are specifically known include N-lithiohexamethyleneimine, n-butyllithium, tributyl tin lithium, dimethylaminolithium, diethylaminolithium, dipropylaminolithium, dibutylaminolithium, dialkylaminoalkyllithium, such as diethylaminopropyllithium and trialkly stannyl lithium, among others.
Chain propagation of an anionically-polymerized polymer typically ceases when all available monomer is consumed or when the living end is quenched or terminated. Typically, termination occurs in the presence of an electrophilic reagent, a terminating agent or a proton donor. Also, living polymers can spontaneously terminate because their carbanion centers decay with time. Spontaneous termination is also prevalent at higher polymerization temperatures where inter-polymer coupling likewise occurs.
The ability to anionically produce living polymers with very narrow molecular weight distributions and then cap the living end, or ends, with a functional group is also generally known in the art. Specifically, termination to produce amines and carboxylic acids has been very successful. Synthesis of polystyrene and polyisoprene containing living polymers with hydroxy (OH) and mercapto (SH) functional end caps were obtained by reacting the living polymer with haloalkanes containing silyl ether and silyl thioether functions (as shown) has also been described as follows. 
The tertiary-butyldimethylsilyl (TBDMS) group is the preferred choice for OH and SH functions in the terminating reactions because the corresponding silyl ethers and thioethers are found to be both stable and compatible with anionic living polymers. A detailed description of the chemistry can be found in the Journal of Macromolecular Chemistry and Physics, xe2x80x9cSynthesis of end-functionalized polymer by means of living anionic polymerizationxe2x80x9d, 197 (1996), pp. 3135-3148.
The rubber compositions can be cured in any conventional manner with known vulcanizing agents. For example, sulfur or peroxide-based curing systems may be employed. For a general disclosure of suitable vulcanizing agents one can refer to Kirk-Othmer, Encyclopedia of Chemical Technology 3rd, Ed, Wiley Interscience, N.Y. 1982, Vol. 20. pp. 365-468, specifically xe2x80x9cVulcanizing Agents and Auxiliary Materialsxe2x80x9d pp. 390-402. The vulcanizing agents can be used alone or in combination.
While the above cited reference employs a sulfur containing reagent in the termination of polystyrene and polyisoprene, the reference only addresses the problems associated with a broad molecular weight distribution. Heretofore, the problems associated with hysteresis and significant improvement in various articles such as tires with hysteresis reduction have not been addressed. That is, the prior art consists of descriptions relating to endcapping of lithium polymers with nitrogen or tin containing reagents. The use of a protected or blocked mercaptan as an endcapping reagent has not been demonstrated as a way of reducing hysteresis.
Thus, a need still exists for methods of preparing polymers and vulcanized elastomers that exhibit reduced hysteresis properties.
In general, the present invention provides a vulcanized elastomeric composition of matter comprising the reaction product of a sulfur-containing reagent with an anionic living polymer said reagent being selected from the group consisting of R3xe2x80x94Sixe2x80x94Sxe2x80x94Rxe2x80x2xe2x80x94X, where R can all be the same or different and is selected from the group consisting of alkyls having from 1 to 8 carbon atoms, cycloalkyls having from 3 to 11 carbon atoms, or aryls having from 6 to 14 carbon atoms, and X is a halogen, to form a R3xe2x80x94Sixe2x80x94Sxe2x80x94Rxe2x80x2 ended polymer; a filler; processing oil; and a cure package containing a deprotecting agent and at least one sulfur cure accelerator.
The present invention also provides a pneumatic tire comprising a vulcanized elastomeric composition of matter with reduced hysteresis properties, wherein the composition comprises a plurality of polymers functionalized with mercapto groups, the mercapto groups in a majority of said functionalized polymers being reacted with unsaturation sites in a polymer backbone, wherein the mercapto groups comprise tert-butyldimethylsilyl-3-chloro-1-propylsulfide.
The present invention provides a method for making a vulcanized elastomeric composition of matter by terminating a living anionic polymer with a sulfur-containing reagent selected from the group consisting of R3xe2x80x94Sixe2x80x94Sxe2x80x94Rxe2x80x2xe2x80x94X, where R can all be the same or different and is selected from the group consisting of alkyls having from 1 to 8 carbon atoms, cycloalkyls having from 3 to 11 carbon atoms, or aryls having from 6 to 14 carbon atoms, Rxe2x80x2 is an alkylene having from 1 to 8 carbon atoms, a cycloalkylene having from 3 to 11 carbon atoms, or an arylene having from 6 to 14 carbon atoms, and X is a halogen, to form a R3xe2x80x94Sixe2x80x94Sxe2x80x94Rxe2x80x2 polymer having a protected mercapto group; deprotecting the mercapto group prior to or during vulcanization of the composition, and attaching the mercapto group to a polymer backbone of one of the polymers.
The present invention also provides a method for reducing hysteresis in an elastomeric composition of matter by deprotecting a R3xe2x80x94Sixe2x80x94Sxe2x80x94Rxe2x80x2 endcapped polymer, where R can all be the same or different and is selected from the group consisting of alkyls having from 1 to 8 carbon atoms, cycloalkyls having from 3 to 11 carbon atoms, or aryls having from 6 to 14 carbon atoms, and Rxe2x80x2 is an alkylene having from 1 to 8 carbon atoms, a cycloalkylene having from 3 to 11 carbon atoms, or an arylene having from 6 to 14 carbon atoms, to form a mercapto functional polymer having mercapto ends; and linking the functional polymer to an unsaturated polymer backbone during or subsequent to vulcanization, such that the resultant elastomeric composition formed includes a plurality of functional polymers having had mercapto ends wherein at least 20 percent are reacted with an unsaturated polymer backbone of the same functional polymer or another of the functional polymers.
This invention advantageously allows a high percentage of the sulfur end caps to be utilized in a hysteresis reducing pathway, particularly when compared to the nitrogen-containing or tin-containing terminated rubbers. The protecting group is removed subsequent to or during vulcanization and then the curatives present in the compound promote a high level (e.g., from about 20 percent up to about 80 percent or more) of the unprotected (free) mercapto groups to react with carbon to carbon double bonds in the polymer backbone. Such a reaction reduces the number of unattached polymer ends present in the vulcanizate and thereby reduces hysteresis (i.e. reduces the tan xcex4).
Advantageously, the use of the reaction product of tert-butyldimethylsilyl-3-chloro-1-propylsulfide with an ionic living polymer as claimed in this invention allows for normal compounding procedures without the risk of premature crosslinking, curing or vulcanization and still allows the polymer endcaps to react with unsaturated sites subsequent to or during the final vulcanization step.
Further, the invention advantageously allows zinc compounds, such as zinc stearate, that are already present in the rubber composition to be used as the means for deprotecting the polymer endcaps alleviating the need for a separate deprotection step.
Also advantageously, the invention can use hydrogen ions or fluorine ions to deprotect the polymer endcaps to allow attachment to the polymer backbone.
As a result of the use of these rubbers, tires and more particularly, tire treads are believed to show improved wear and vehicles using such tires are believed to show improved fuel efficiency.
It should become apparent from the specification that follows that one or more of the foregoing advantages obtained by this invention over the prior existing art, can be accomplished as described and claimed herein.
As noted hereinabove, the present invention is directed toward the preparation of low hysteresis rubber. More specifically, the invention is directed toward the preparation of low hysteresis rubber by terminating an anionically-initiated lithium polymer with a sulfur containing reagent, namely tert-butyl dimethylsilyl-3-chloro-1-propylsulfide, and subsequent reaction of the potential mercaptan end group to produce a mercapto end group which can then react with the unsaturation in the polymer backbone of one of the polymers employed.
The anionically-initiated lithium polymer is defined as a xe2x80x9cliving polymer.xe2x80x9d Such a living polymer of the present invention has the general formula, prior to quenching, of
polymer-Li
where the polymer is any diene homopolymer, diene copolymer, aromatic homopolymer, diene/monovinyl aromatic random copolymer or unsaturated elastomer. The lithium proceeds to add monomers to grow the chain as polymerization continues, until the reaction is quenched or terminated. Scheme I shows initiation of polymerization using an organolithium initiator. 
In Scheme 1, R is selected from the group consisting of functionalized and non- functionalized lithium-organo groups (e.g., n-butyl lithium) while P is the repeat polymer chain, butadiene-styrene.
Polymerization is usually conducted in a conventional solvent for anionic polymerizations such as hexane, cyclohexane, benzene and the like. Other techniques for polymerization, such as semi-batch and continuous polymerization may be employed. In order to promote randomizing during copolymerization and to increase vinyl content, a modifier may optionally be added to the polymerization ingredients. Amounts range between 0 to about 90 or more equivalents per equivalent of lithium. The amount depends on the vinyl content desired, the level of styrene employed and the temperature of the polymerizations, as well as the selected modifier.
Compounds useful as modifiers are organic and include those having an oxygen or nitrogen hetero-atom and a non-bonded pair of electrons. Examples include dialkyl ethers of mono and oligo alkylene glycols; xe2x80x9ccrownxe2x80x9d ethers; tertiary amines such as tetramethylethylene diamine (TMEDA); THF; THF oligomers; linear and cyclic oligomeric oxolanyl alkanes, and the like. Details of linear oligomeric oxolanyl modifiers can be found in U.S. Pat. No. 4,429,091, owned by the Assignee of record, the subject matter of which is incorporated herein by reference.
Polymerization is usually begun by charging a blend of the monomer(s) and solvent to a suitable reaction vessel, followed by the addition of the modifier and the initiator solution. Alternatively, the monomer and modifier can be added to the initiator. The procedure is carried out under anhydrous, anaerobic conditions. The reactants are heated to a temperature of from about 30xc2x0 to about 120xc2x0 C. and are agitated for about 0.15 to about 24 hours. After polymerization is complete, the product is quenched or terminated.
Quenching is usually conducted by stirring the polymer and quenching or terminating agent for about 0.05 to about 2 hours at temperatures of from about 30xc2x0 to 120xc2x0 C. to ensure complete reaction.
Lastly, the solvent is removed from the polymer using coagulation with water, alcohol or steam. If coagulation with water or steam is used, oven drying may be desirable.
The present invention is preferably terminated with a sulfur-containing terminating agent. The preparation of such a sulfur terminated rubber from the reaction with a lithium polymer produced a rubber having both a reduced number of non-reactive ends and an increase of potentially reactive ends. The latter are activated by removing the silyl blocking agent during cure to give a reduced level of hysteresis. It has been found in this invention that the use of a protected or blocked mercapto group as an endcapping reagent reduces hysteresis in vulcanizable compositions of matter that are useful for making tires, and is expected to provide tires and tire components having decreased hysteresis properties without substantially affecting the mechanical, wear, and tear strength of the tire rubber. Accordingly, the present invention contemplates vulcanized compositions of matter, tire recipes, belts and tire components containing mercapto functionalized polyolefin. The invention also contemplates the method of manufacture of the same.
Illustrative examples of useful polyolefin rubbers include, but are not limited to homopolymers and copolymers of isoprene and butadiene, such as polyisoprene and polybutadiene and poly(butadiene-isoprene), and, copolymers and terpolymers of styrene, butadiene and isoprene such as poly(styrene-isoprene), poly(styrene-butadiene) (SBR), poly(butadiene-styrene-isoprene) and combinations thereof. Any unsaturated (polymer containing unsaturation) useful in the manufacture of vulcanizates is useful in terms of the present invention. Scheme II shows terminating the living polymer with a sulfur containing reagent, namely tert-butyldimethylsilyl-3-chloro-1-propylsulfide, to form the protective functional endcap for the polymer which, in this scheme, is shown to be an SBR copolymer. 
The tert-dimethyl silyl propylsulfide endcap is relatively unreactive which allows for increased processability without premature crosslinking or vulcanization. Moreover, when compared to the nitrogen or tin containing terminated rubbers of the prior art, this invention has the advantage of allowing a very high percentage of the sulfur endcaps to be utilized in the hysteresis reducing reaction pathway. In the instant case, prior to vulcanization or during the step of curing, and particularly sulfur curing, the protecting tert-dimethyl silyl group can be removed by using a deprotecting agent selected from the group consisting of additives containing H+, additives containing Fxe2x88x92, and zinc compounds. It will be appreciated that additives containing the hydrogen or fluorine ions can be used to provide the necessary deprotecting capabilities of the ions. Because zinc stearate is already generally present in vulcanization cure packages it is the preferred deprotecting agent of the invention. Zinc stearate allows deprotection without adding a deprotection step to the manufacturing process. Stoichiometric amounts of the deprotecting agent are employed. Scheme III shows deprotecting the functional endcap of the quenched polymer. 
Prior to vulcanization, and preferably during vulcanization, the tert-butyl dimethyl silyl protective endcap is removed. Then, during or subsequent to the curing process, the curatives present in the compound promote a high level of the exposed, unprotected-SH mercapto functional endcaps to react with or attach to the unsaturation points in the unsaturated polymer backbone. Notably, at least 20 percent and preferably, a majority of the mercapto ends are reacted with the unsaturation sites in the unsaturated polymer backbone. More preferably, at least 70 percent of the mercapto ends are reacted, and testing has shown that up to about 80 percent of the mercapto ends can be reacted. The carbon to carbon double bond unsaturation points may be located on the same polymer chain as the functional endcap or on another polymer chain. The vulcanization step proceeds as known in the art. Scheme IV shows mercapto endcaps crosslinked to an unsaturated polymer backbone.
In the preceding Scheme, the R and P groups are as previously designated, while the Pxe2x80x2 and Pxe2x80x3 groups represent another unsaturated polymer backbone, i.e., any of the unsaturated polymers described hereinabove. While Pxe2x80x2 and Pxe2x80x3 can be the same or different backbones, the Pxe2x80x2 group is one in which the mercapto group has reacted with a site of unsaturation. The Pxe2x80x3 group designates one in which the mercapto end cap of that backbone has reacted with the site of unsaturation from a previously reacted mercapto end group polymer backbone.
In light of the foregoing, it will be appreciated that the reaction of the unprotected mercapto end groups to the polymer backbone reduces the number of unattached polymer ends present in the vulcanized rubber, thereby reducing the tan xcex4 of the composition and reducing the hysteresis of the composition.
In order to form vulcanized elastomeric compositions, the polymers of the present invention can be vulcanized with conventional vulcanizing agents, such as sulfur and accelerators. The amount of the agent used is 0.1 to 5 parts by weight, preferably 0.1 to 3 parts by weight, based on 100 parts by weight of the rubber material, with a range of from about 0.1 phr to about 2 phr being preferred. When the amount is more than 5 parts by weight, the rubber elasticity is lost.
Representative of conventional accelerators are amines, guanidines, thioureas, thiols, thiurams, sulfonamides, dithiocarbamates and xanthates which are typically added in amounts of from about 0.2 to about 10 phr, with a range of from about 2 phr to about 5 phr being preferred. Representative of sulfur vulcanizing accelerators include TMTD (tetramethylthiuram disulfide), CBS (N-cyclohexyl-2-benzothiazole sulfenamide), MBT (mercaptobenzothiazole)and mixtures thereof.
The elastomer compositions may also contain conventional additives including reinforcing fillers and non-reinforcing fillers, peptizing agents, pigments, stearic acid, antiozonants, antioxidants, processing oils, activators, initiators, plasticizers, waxes, prevulcanization inhibitors, extender oils, waxes, and the like. Representative of reinforcing agents include carbon black, which is typically added in amounts ranging from about 5 to 100 parts by weight based on 100 parts by weight of total rubber (phr). Preferably, carbon black is used in amounts ranging from about 15 to 85 phr. Typical carbon blacks that are used include N110, N121, N220, N231, N234, N242, N293, N299, N326, N330, N332, N339, N343, N347, N351, N358, N375, N472, N539, N550, N660, N683, N754, and N765. Depending on the particular use of the compound, the appropriate carbon black may be selected.
Typical filler materials also include reinforcing and non-reinforcing fillers conventionally used in vulcanizable elastomeric compounds such as clays, talcs, mica, calcium carbonate, silica and other finely divided mineral materials. Selection of the filler material(s) (mixtures) is not critical to practice of the present invention.
Representative of the antidegradants which may be in the rubber composition include monophenols, bisphenols, thiobisphenols, polyphenols, hydroquinone derivatives, phosphites, phosphate blends, thioesters, naphthylamines, diphenol amines as well as other diaryl ainine derivatives, paraphenylene diamines, quinolines and blended amines. Antidegradants are generally used in an amount ranging from about 0.1 phr to about 10 phr with a range of from about 0.5 to 6 phr being preferred.
Representative of a peptizing agent that may be used is pentachlorophenol which may be used in an amount ranging from about 0.1 phr to 0.4 phr with a range of from about 0.2 to 0.3 phr being preferred.
Representative of processing oils which may be used in the rubber composition of the present invention include aliphatic-naphthenic aromatic resins, polyethylene glycol, petroleum oils, ester plasticizers, vulcanized vegetable oils, pine tar, phenolic resins, petroleum resins, polymeric esters and rosins. These processing oils may be used in a conventional amount ranging from about 0 to about 50 phr with a range of from about 5 to 25 phr being preferred.
Zinc oxide and stearic acid are conventionally used to vulcanize elastomers. Zinc oxide is generally used in a conventional amount ranging from about 0.5 to about 5 phr. Stearic acid is generally used in a conventional amount ranging from about 1 to about 4 phr.
The practice of the present invention is especially useful in general rubber recipes, but inasmuch as the decrease in hysteresis properties does not deleteriously impact the wear, mechanical, and tear strength of the rubber, the practice of the present invention may also be applied to the tread and sidewall stocks of pneumatic tires. Furthermore, it should be understood that the practice of the present invention is believed to be especially advantageous for off-road or heavy-duty truck tires. The practice of the present invention will also improve other tires, for example passenger tires.